text:
How much energy does unretracted landing gear consume?
I am interested in finding energy consumption (% kN) of unretracted landing gear  (on a jet fighter) per hour. Although the shape and size tell us how much drag is being produced I need a general rule of thumb.

FMI, what is “kn”?

@Michael Hall, Kilo Newton

While it’s not a true air to air fighter, following are some comparison numbers for the EA-6B Prowler that might help provide a useful ratio for a tactical "fighter type" aircraft.  (Information comes from the NATOPS pocket checklist, under the constant altitude “bingo” fuel divert tables.)
Theses figures are for a flaps-up aircraft, fully loaded with 5 external stores, and both engines operating.  (There are multiple configurations and distances covered in the tables).
Fuel required to go 100 Nautical Miles:

Altitude
Gear Up
Gear Down

Sea Level
3970 lbs
5220 lbs

10,000 ft
3580 lbs
4590 lbs

20,000 ft
3270 lbs
4160 lbs

NOTE:  The gear down figures are worst case and presume a hydraulic failure.  In the event of a hydraulic failure the gear will be blown down pneumatically and the forward main landing gear doors, (that normally re-close to cover the empty wheel well) will remain open.  This imposes some additional drag penalty over and above normal flight with the landing gear down.

You should be able to extrapolate this into something useful, but if you would like more data points please let me know.

Fuel has fixed amount of energy per pound, so just divide the two numbers for matching cases. Result is about 30% more power with gear.

@Jan Hudec, 30% more power means if a plane is moving with 500 KN of thrust with retracted landing gear , it will use ~650 KN thrust?

@Thomas as rough estimate, yes. The fuel required table probably assumes you fly slower with the gear extended, which complicates things quite a bit.

I remember Terry telling us once he had to fly a 747 with gear down and used twice the fuel.

@Thomas make it over at https://physics.stackexchange.com/; nothing specific to aircraft really, just some basic dynamics.

@Jan Hudec, ok and you're right but what does AVIATION "  mean?? Also you say the basic are answered here but anyone explains in a way that all of them are PHD. Apart from that could you answer my question?

@Thomas, it will definitely take more than comments—and someone with better idea about behaviour of nozzles.

https://aviation.stackexchange.com/a/15988/66887
Terry said twice fuel usage

It is doubtful whether you will find any data beyond "a lot". This is the reason for retracting the gear whenever possible. A while ago an Airbus A320 was unable to retract the gear. The pilots didn't find specific data in their manuals. The additional fuel use resulted in the plane never flying again.
https://en.wikipedia.org/wiki/Hapag-Lloyd_Flight_3378

Welcome to Aviation.SE! While I agree with your post, it would be better suited as a comment. Also, HF3378 was an A310, not A320.

Also see https://onemileatatime.com/air-india-plane-diverts-after-pilots-forget-to-retract-landing-gear/

Anecdotally, a glider with and without retractable undercarriage (typically just one wheel, and only partly exposed) changes the glide ratio from 70:1 to 100:1

@Bianfable didn't get your meaning of suited as a comment

@Thomas, he means that it isn't an answer to the question.  Which is isn't.  It is a comment.


text:
Is the elevator really an 'angle of attack control' as described in Stick and Rudder?
I've been reading Wolfgang Langewiesche's book Stick and Rudder and am a little confused about the definition of the elevator as the "Angle of Attack Control" or rather, the idea that each position of the elevator corresponds to a unique angle of attack.
I was under the impression that Angle of Attack is the angle the wing meets the relative wind. Consider a certain elevator position for straight and level flight at cruise rpm. If I then add power and climb (while holding the same attitude) isn't it true the Angle of Attack will increase because the relative wind is now coming from in front and above the plane? If this is true, then how can I reconcile this with the idea the elevator is an "angle of attack" control. Isn't the plane's Angle of Attack based on not just the position of the elevator, but the motion of the plane through the air?
If anyone could give me some advice to clear this up it would be greatly appreciated!

Didn't you mean to say "...isn't it true that the Angle of Attack will decrease...?"

Highly related -- https://aviation.stackexchange.com/q/64579/34686

Wolfgang Langewiesche is right to appropriate order of approximation. For every elevator position a statically stable airplane settles to a specific equilibrium angle of attack. That's how static stability works.
Airplane is statically stable if and only if increasing angle of attack causes higher increase in coefficient of lift on the aft airfoil (tail for normal layout). Then increasing angle of attack (by random fluctuation like turbulence) increases the lift aft more, the overall centre of pressure moves aft and the aircraft pitches nose down to return the angle of attack to the trimmed position.
By moving the elevator you adjust the coefficient of lift on the control surface, and the plane needs to assume a different angle of attack to shift the centre of pressure back to coincide with centre of gravity. Most aircraft are designed so this is quick enough that if you don't jerk the controls too much the angle of attack simply follows the elevator position.

isn't it true the Angle of Attack will increase because the relative wind is now coming from in front and above the plane?

The angle of attack is high when the relative wind is from below and low when it is from above.

If I then add power and climb (while holding the same attitude)

If you add power and leave the yoke as trimmed, the plane will start to accelerate, which increases the lift. That causes the plane to accelerate upward, which decreases the angle of attack. This decreases the lift more on the tail, so the tail sinks and the aircraft pitches up until it assumes the original angle of attack. At this point, the pitch angle is increased by the same amount as the flight path angle.
The aircraft is still moving faster than the original speed due to inertia, so the lift is still more than weight and the aircraft continues accelerating up and pitching up to maintain the angle of attack. When the speed returns to the trimmed, the aircraft is already pitched too much, so now it will slow down and pitch down again and repeat the cycle. This is called the Phugoid oscillation. In most planes it is damped, but still annoying if you don't counter it with a bit of elevator deflection. But if you don't touch the trim, just arrest the phugoid, the aircraft will settle in climb at the same speed.
It has some consequences:

Tendency of the plane to pitch down means it is losing airspeed.
If you are not pulling on the yoke, or trim aggressively nose up, you won't stall. Unfortunately pulling on the yoke to keep the nose up becomes too much of a muscle memory so it's easy to miss this clue.
It is how pitch for speed and power for descent rate works on descent.

See also How It Flies, specifically section 2.3 talks about trim and angle of attack, with more details further in chapter 6.

Stick and Rudder also states on page 5 that we normally teach "theory of building the airplane rather than of flying it." Take the Lift Formula, and let's put it into terms the pilot can control in flight. Lift = coefficient of lift x 1/2 air density x true velocity squared x wing area.
When we teach this in ground school, we instantly see the eyes glaze over on over half of the class. Simplified for Pilots, the Lift formula can be shown as
Lift = (better shown as proportional to) Angle of Attack x KIAS x KIAS.
Now we have only two items that enter into the formula: AoA and the Airspeed dial.
(wing area can't be controlled by the pilot in flight, unless flying something like the F-111 with a wingsweep handle and huge double-slotted Fowler Flaps). True Airspeed Velocity and air density interact to get to Calibrated Airspeed (KIAS when adjusted for installation), and the Pilot's handbook of Aeronautical Knowledge (PHAK) figure 5.5 shows a straight line relationship between Coefficient of Lift and Angle of Attack.
Therefore, when in the cockpit, the only way the pilot can control lift is by adjusting either AoA or KIAS.
Note: Throttle does not enter into the lift equation, unless the pilot is trying to maintain a specific airspeed while maneuvering, which requires throttle movement to counteract changes in drag.
Note: Trim does not enter into the Lift equation. All that trim accomplishes is to reduce/eliminate stick/yoke pressure to maintain a specific hands-off angle of attack (which some equate to attitude or airspeed, but trim really just commands a hands-off AoA).
Langewische is correct about elevator position (which equates to stick/yoke position) commanding a specific AoA. But, as always there are some caveats such as:

Center of Gravity Location (forward vs rearward CG within range)
Aircraft configuration (flaps, gear, etc)
Power downwash over the tail.

So, we can't just paint marks on the control yoke rod "green, yellow, red."
There was some talk earlier saying "what can happen if the engine abruptly quits while an aircraft is in a very steep climb". What will really happen is the airplane will rotate downward to maintain the trimmed AoA and will not stall "unless" the pilot continues to pull the yoke backward (which is what nearly always happens in this stressful situation).
There were some other comments about flying a phugoid to show stability. The pitch phugoid is great to show stability, but it also shows the relationship between AoA, speed, lift, and stick/yoke position.
Here's a great demonstration, which is best accomplished in an airplane with both a G-meter and and AoA gauge. Trim the airplane for 1-G, steady speed, level flight. Slow the airplane to a range where not all of the green bars on the AoA gauge are illuminated (I do this around 80-90 KIAS). Pull the nose about 20 degrees nose up, then take your hand off the stick/yoke. Monitor the yoke position--it will revert back to the same position it was before you pulled (I've used a ruler to document stick position). Watch the AoA gauge (it will remain nearly constant), watch the stick/yoke not move, watch the G-meter go to less one when the airspeed is slow (at the top) and more than one when the airspeed is high (as the airplane pulls out). This is because the amount of lift required is constantly changing (L = AoA x KIAS x KIAS), but the AoA is not changing (or changing slightly, due to mostly to momentum).
As for maneuvering flight, the lift equation remains the same. The airplane doesn't care what attitude it is in, just how much lift is required. When you roll into that 60 degree bank (LEVEL, 2G turn, while maintaining constant speed with throttle), the airplane sees a need to generate twice the amount of lift. Since airspeed is unchanged, the AoA must increase to double the lift--this is done by pulling back on the stick/yoke a specific amount to command that AoA.
Is there some lag? yes, Momentum enters into the equation a bit, but the plane still reacts pretty quickly to elevator inputs (especially when at higher speeds). Can you pull so abruptly that things get out of kilter a bit, yes.
Another great demonstration is to get a flight in an Ercoupe, especially an older one with just a one brake pedal on the floor. Slow up, pull the yoke all the way back, you will not stall (maybe you can get close with a super abrupt pull, but then the plane will stabilize out). You can get into a pretty good rate of descent, but the elevator will not travel high enough to maintain a stall.
So, knowing about the physical position that the stick/yoke will reach stall AoA (in the configuration, CG, etc., that you typically fly) will give you another indication of when you are approaching a stall in either 1G or accelerated G flying. (You don't have to know that position exactly, just be aware of the approximate location, kind of like playing a trombone).
I'm sure this will be controversial, but I hope you will think about these comments and try out the maneuvers before commenting. This is not meant to be a discussion on how to build an airplane (the actual aero formulae get extremely complicated, and heck, we can't agree on whether Bernouli, Newton, or Coanda develop lift), but this is meant how to provide tools to the pilot understand what is happening in flight that they have "control" over.

Re "Watch the AoA gauge (it will remain nearly constant)"-- I've done similar experiments in Cessna 152s and 172s and was consistently able to trim the airplane so that in hands-off flight, the stall horn consistently sounds at the top of each pitch phugoid oscillation but is quiet at the bottom of each pitch phugoid oscillation. Food for thought. My interpretation is that the curvature of the flight path in the pitch dimension causes a "damping" effect on the tail surfaces that tends to decrease the AOA of the wing whenever the curvature of the flight path is in "upward" direction rel to a/c.

Which I interpret as being a bit different than "momentum". Would be interesting to really try to evaluate the relative importance of pitch rotational inertia (momentum), versus "damping" effect due to curvature of flight path/ relative wind. Another way to look at "damping" is that curving relative wind is pushing "down" on tail whenever flight path is curving "upward"; this effect is very pronounced in hang gliders due to low flight speed and resulting small radius of curvature of flight path while maneuvering, even though their pitch rotational inertia is rather low.

In both hang gliders and sailplanes, the pilot typically holds a strong nose-up pitch control in a steady-state moderate-to-steep-banked turn while circling at min sink speed, such as would be sufficient to produce a full stall if the wings were level.  Since the turn is a steady-state manuever, we can eliminate momentum/inertia as a consideration here.  So the pitch "damping" phenomenon due to non-zero pitch rate (i.e. due to curvature of flight path in pitch axis-- two sides of the same coin, at least in a steady-state maneuver) must be the key reason that this pitch input is required.

But to a first approximation the observation that the a-o-a remains nearly constant through the phugoid is a valid one--

It seems to me that the crux of the misunderstanding in your question is here:

If I then add power and climb (while holding the same attitude)

If you hold the elevator in a fixed position and add power, the airplane will pitch up relative to the ground, to the extent needed to preserve the original angle of attack relative to the airplane's motion through the air (i.e. "relative wind").
If by "holding the same attitude", you mean "maintain the elevator in a fixed position", then your answer is in your question. Angle of attack doesn't change if you don't change the elevator position; if in your question you mean to assert the latter, then the former results, just as the book says.
On the other hand, if you mean "maintain the airplane's orientation relative to the ground", then the only way to accomplish that after you add power is to change the elevator position, reducing actual angle of attack. This negates the premise of your question.
It's not entirely clear from your post which you mean, but either way it's interpreted, the resulting explanation is consistent with the information you're asking about.

There is a contradiction built into the question.  You write:

Consider a certain elevator position for straight and level flight at
cruise rpm. If I then add power and climb (while holding the same
attitude) isn't it true the Angle of Attack will increase because the
relative wind is now coming from in front and above the plane? If this
is true, then how can I reconcile this with the idea the elevator is
an "angle of attack" control?

You've implied that you've kept the elevator position constant as you added power and climbed.  You've also stated that the aircraft's attitude has remained constant.  You've over-constrained the problem.  The elevator is to a first approximation an angle-of-attack control, not a pitch attitude control, and it's not possible for the angle-of-attack and the pitch attitude to remain constant as the plane transitions from level flight to a climb.
No doubt if we tried hard enough we could come up with an exotic example where some particular aircraft could in fact maintain both a constant elevator position and a constant pitch attitude when power was added to transition from level flight to a climb.  For example, if the nose-mounted engine had a great deal of downthrust, causing a large decrease in angle-of-attack when the power was added and the elevator position was held constant.  Most pilots would consider such an aircraft to be unpleasant and unnatural to fly.  To a first approximation, in most aircraft the angle-of-attack is primarily determined by the position of the elevator.  That's what the elevator is designed to do-- to control the angle-of-attack of the wing.
It's often true that the power setting has some influence on angle-of-attack, for a given elevator position, but this is generally a byproduct of the thrust line location and other aspects of the aircraft's geometry, not an intentional design feature.  Designers typically strive to minimize such effects.
Langewiesche's basic point is that if the control stick or yoke is not pulled too far aft, the wing cannot stall, regardless of the aircraft's pitch attitude.  This isn't always exactly true-- consider what can happen if the engine abruptly quits while an aircraft is in a very steep climb, or if the towline breaks while a glider is climbing very steeply on a winch tow.  But it's a good starting point.  To a first approximation, the elevator controls the angle-of-attack of the wing, not the pitch attitude of the aircraft.  The pitch attitude is a result of the angle-of-attack of the wing and the climb or descent angle, and the angle-of-incidence at which the wing is attached to the fuselage.
We often hear that "pitch plus power equals performance".  And pilots often say things like "pitch for a climb at Vy".  Statements like these may seem to carry some implication that the elevator is controlling the aircraft's pitch attitude.  While it's certainly true that changing the elevator position will change the aircraft's pitch attitude, it's also true that as we vary the power setting from idle to full power, we'll find that any one particular elevator position correlates to one specific angle-of-attack much more closely than it correlates to any one particular pitch attitude.  This also means that as we vary the power setting from idle to full power, we'll generally find that there's a good correlation between elevator position and airspeed, at least for any given G-loading.1
Footnotes:

At really extreme climb or descent angles, the correlation between angle-of-attack and airspeed changes, which also means that the correlation between elevator position and airspeed changes.  At really extreme climb or descent angles, the airspeed indicator no longer serves as an "angle-of-attack gauge" in the same way that it does when the flight path is closer to horizontal.  This is discussed in more detail in this related ASE answer to a related question -- which also has some other content that you may find relevant to your question.

For a given airspeed, there is exactly one distinct angle of attack possible resulting in straight stabilized flight. If it would be higher or lower in any moment, there is excessive or insufficient lift and forces are no longer in balance. It gets "adjusted" thanks to airplane designed longitudinal stability by either readjusting change in pitch, changed airspeed or rate of climbing, likely more of these simultaneously.
So you can think -- for straight flight -- that setting AoA is the same as setting airspeed. Maybe it is more convenient way? (In steep high-G turns this relation breaks as you need higher airspeed for given AoA to provide enough lift. Here, a properly working trim should hold stable AoA and let airspeed increase.)
Another fact to keep on mind is that, in first order approximation (neglecting extreme attitudes) airplane's stability and pitch control does not care too much about horizon and attitude. It is AoA which defines forces regardless if it is affected by attitude or climb. (The change in direction of gravity force has usually negligible effect here.) Elevator or trim has no direct way to "feel" attitude w.r.t horizon, so that's why controlled value is AoA.
On the other hand, you engine does care a lot about vertical speed because of available power. If you trim for specific AoA (or speed), then depending on available power, it will result in horizontal flight, or climb with same AoA (and therefore more nose-up attitude to compensate for change in relative airflow), or sink with more nose-down attitude.

If you add power while holding the same attitude you will accelerate.  As you accelerate you will have to hold the nose down and trim, (gradually changing your attitude) but this will actually decrease rather than increase the AOA.
Unless you want to climb.  If you add power and pitch up to hold the same airspeed, you will climb at the same AOA and airspeed that you previously had during level flight.
The relative wind in a climb is coming more from above, (which would otherwise decrease, not increase AOA) but because you pitched up the angle of that relative wind to the wing chord can, and does, remain the same for a given airspeed.
Since the elevator controls pitch it is also correct to say that it directly controls AOA as well, although you are correct that it is also dependent on the aircraft's motion through the air.  (think of a loop, with positive AOA at the top...)

Your understanding of AoA is correct.  While the elevators do influence AoA they don’t control it, so you’re right that a particular elevator position doesn’t correspond to a particular AoA.  You example is flawed though - if you increase power to climb then your AoA might not change at all, or not significantly.

I'm not sure what you mean by "influence", but the elevator most definitely controls AOA.

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text:
How can a glider pilot add sport pilot privileges?
If I understand the rules correctly, if one already has a Private Pilot rating, then one has Sport Pilot privileges but will need an endorsement for a new category/class of aircraft. What if one has a Private Glider rating? Would that person be able to fly a Sport Pilot power aircraft with just an endorsement?

Welcome to aviation.SE! Can we assume that you're asking about FAA regulations?

Yes. My situation is that I have a Private Pilot - Glider certificate (over 3000 hours). Have held a class III medical in the past and have some solo hours in single engine aircraft (J3 Cub, C-140, C-150/152, C-175). Never completed the Private Pilot  - airplane due to life/job/$$$). I qualify for Basic Med and am working on the Private Pilot - airplane cert. I just bought a TL-2000 Sting light sport plane. Several people told me I should be able to get the Sport Pilot -airplane privileges through the endorsement process rather than having to take the written exam.

Based on your current situation, the answer is yes with caveats. You only need one endorsement every 90 days from your CFI to fly solo in your TL-2000. As a matter of fact, the endorsement is make and model specific. So, your CFI could endorse any model in which you are trained. There will be limits on the operations you may fly. One big limitation would be that you can not fly passengers as PIC.

By the way, you only need to pass the written knowledge exam once in your life for each Pilot Certificate regardless of how many Category, Class, and Type ratings that you get. You pass a different knowledge test for an instrument rating. You may have to pass different Practical Exam Checkrides for different Cat, Class & Types. The only exception to this is the ATP. You need a written to add Category and Class ratings for ATP.

If you have a PPL certificate for Single Engine Land you can fly any of the Single Engine Land Sport Aircraft since they are in the same category and class. You do not need any additional endorsements or ratings. It woud be a good idea to get training in the specific aircraft singe they may handle slightly differently than what you are used to  because of their smaler size and weight.

@JScarry - That is somewhat correct for ASEL. Although, not every ASEL would qualify for your statement. An ASEL may require a type rating to operate. The same is true for high powered, complex, and tail wheel ASELs. They would require a specific endorsement to operate. Although, there wouldn’t be any complex nor high powered LSAs. What if the private pilot does not have a powered aircraft category and class rating as is the case for the glider pilot who originally posted the question?

@DeanF. If you have an ASEL PPL then you automatically qualify to fly single-engine land sport pilot. If you have additional endorsements or type ratings, that’s fine but it doesn’t apply to sport pilots since —by definition—none of the sport planes are complex or high performance. A glider pilot does not have an ASEL PPL certificate so cannot fly a sport plane.

@JScarry - My thoughts exactly. Although, I don’t see a way around the taildragger endorsement even in an LSA. With as many flight hours as the glider pilot who posted the question has, hopefully he has finished his PPL-ASEL under Basic Med by now.

@DeanF. I had forgotten that there are taildraggers that qualify under the sport pilot rules. You would need an endorsement to fly them.

The short answer is no. You can get student pilot privileges with only a CFI endorsement for a new Category and Class. You would not have Sport Pilot privileges, though. The regs for Sport Pilot certificate holders would be that you need endorsements from two different CFIs and file an application with the FAA for each Category and Class.
Although 61.303 and 61.321 can be confusing, they both make mention of category and class. A sport cert does not have Cat & Class. For Sport Pilot privileges you have to have a training endorsement from one instructor and a proficiency check endorsement from a second instructor before the FAA will issue you a Cat & Class endorsement.

61.321 How do I obtain privileges to operate an additional category or class of light-sport aircraft?
If you hold a sport pilot certificate and seek to operate an additional category or class of light-sport aircraft, you must—
(a) Receive a logbook endorsement from the authorized instructor who trained you on the applicable aeronautical knowledge areas specified in §61.309 and areas of operation specified in §61.311. The endorsement certifies you have met the aeronautical knowledge and flight proficiency requirements for the additional light-sport aircraft privilege you seek;
(b) Successfully complete a proficiency check from an authorized instructor other than the instructor who trained you on the aeronautical knowledge areas and areas of operation specified in §§61.309 and 61.311 for the additional light-sport aircraft privilege you seek;
(c) Complete an application for those privileges on a form and in a manner acceptable to the FAA and present this application to the authorized instructor who conducted the proficiency check specified in paragraph (b) of this section; and
(d) Receive a logbook endorsement from the instructor who conducted the proficiency check specified in paragraph (b) of this section certifying you are proficient in the applicable areas of operation and aeronautical knowledge areas, and that you are authorized for the additional category and class light-sport aircraft privilege.

Private, Sport, Recreational, Commercial, etc. are all separate certificates. You can have multiple category and class ratings on each. If you have a Private certificate already, you will not need to get another certificate for any further non-commercial privileges. All you need to add a different Category or Class is an endorsement showing appropriate training and a Checkride. You will retain Private Privelages for any Category and Class listed on your certificate. Glider is a separate Category to any powered aircraft.

61.63 Additional aircraft ratings (other than for ratings at the airline transport pilot certification level).
(a) General. For an additional aircraft rating on a pilot certificate, other than for an airline transport pilot certificate, a person must meet the requirements of this section appropriate to the additional aircraft rating sought.
(b) Additional aircraft category rating. A person who applies to add a category rating to a pilot certificate:
(1) Must complete the training and have the applicable aeronautical experience.
(2) Must have a logbook or training record endorsement from an authorized instructor attesting that the person was found competent in the appropriate aeronautical knowledge areas and proficient in the appropriate areas of operation.
(3) Must pass the practical test.
(4) Need not take an additional knowledge test, provided the applicant holds an airplane, rotorcraft, powered-lift, weight-shift-control aircraft, powered parachute, or airship rating at that pilot certificate level.
(c) Additional aircraft class rating. A person who applies for an additional class rating on a pilot certificate:
(1) Must have a logbook or training record endorsement from an authorized instructor attesting that the person was found competent in the appropriate aeronautical knowledge areas and proficient in the appropriate areas of operation.
(2) Must pass the practical test.
(3) Need not meet the specified training time requirements prescribed by this part that apply to the pilot certificate for the aircraft class rating sought; unless, the person only holds a lighter-than-air category rating with a balloon class rating and is seeking an airship class rating, then that person must receive the specified training time requirements and possess the appropriate aeronautical experience.
(4) Need not take an additional knowledge test, provided the applicant holds an airplane, rotorcraft, powered-lift, weight-shift-control aircraft, powered parachute, or airship rating at that pilot certificate level.

For instance, I have a Private Cert for Airplane Single Engine Land. I also have Rotorcraft Helicopter hours. I can not solo a helicopter without an endorsement from my instructor. Essentially, giving me Student Pilot privileges for helicopters without a student pilot license (minus the Class B restriction). I would need a Checkride to add Rotorcraft Helicopter to my Private Cert.

I don't believe this is completely accurate in the case of sport pilots. AFAIK, categories and classes don't apply directly to sport pilots. See 61.303, which is probably what the OP is looking for. And all of this assumes the OP is indeed asking about the US, of course.

@Pondlife - I figured as much. Although 61.303 and 61.321 can be confusing, they both make mention of category and class. A sport cert does not have Cat & Class. You have to have an endorsement from one instructor and a proficiency check from a second before the FAA will issue you a Cat & Class endorsement.

@Pondlife - if you only have a Private cert with only a glider rating, chances are that you do not have a Med Cert. It is not required. In which case, you only have privileges for gliders and balloons of a sport cert. Otherwise, you have to follow the process in the pre ious comment

I agree, sport pilot stuff is typically a bit messy - or less understood! - than the rest. I was mostly commenting on the wording "You can have multiple category and class ratings on each", which isn't accurate for RP or SP, at least as far as I know. As always, I'm completely prepared to be wrong :-)

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text:
How has FAR 61.57 changed since 1969?
An answer to another question on Aviation.SE asks about the historical development of FAR 61.57.
How has FAR 61.57 changed over the years?

Glad you asked!
I referenced historical FARs from 2013, 1993, 1988, and 1969.  Here's what I found:

Why yes, it has, and the history along with the preambles and lots of other good information is available on the FAA's Regulatory Guidance Library.
Click the image and then click on Historical CFR and then By Part and follow the tree to 61.57 to see the history:

More details are available here:  How can I see the revision history of a particular FAR?

Huh.  That would have saved me some gas.  Curses.

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text:
May a student continue a discontinued checkride if the CFI who endorsed them voids that endorsement?
The Situation
CFI Charlie endorses Student Sara's logbook for a checkride. Sara begins the checkride but discontinues, and receives a letter of discontinuance. So far so good.
Before Sara can reschedule the rest of the checkride, she and Charlie have a disagreement. Charlie has access to Sara's logbook and voids his endorsement by crosshatching the entire text of the endorsement before giving it back to her.
The questions

Is Charlie's endorsement actually considered void?

Sara has the letter of discontinuance in hand. Can she reschedule and complete the checkride without getting a new endorsement?

Sadly, a true (and very unprofessional) story.  Names have of course been changed.

Wow that musta been some argument...

I think CFI Charlie way overstepped his bounds. Nothing in the FAA rules or Circulars talk about revoking endorsements anywhere. I've looked.

I would say that Sara just needs to present the log to her examiner and say "now what", and let the FAA guy sort it out.  What else can she do?

From what I understand, the CFI, after signing off, has no more supervisory role in the checkride process. Claiming to do so might involve the FSDO.
It has been argued elsewhere that scratching off an otherwise valid signature would be an act of vandalism.
These days however, the CFI needs to register that in a Database with the FAA in leu of an actual signature, which would be the gold standard
lastly, talk to the DPE, if they say yes, continue the checkride, get your licence, and the question is moot.

I think whether CFIs can generally retract an endorsement is a different question than this one.  A discontinuance to a check-ride is simply indicating that the event will be completed in separate sessions, but as far as the checkride status is concerned, it is still open/ongoing.  Think of it like this: what would happen if the CFI barged in the room after the oral was passed and demanded that the student not be allowed to continue the flight portion and verbally retracted her endorsement to take the checkride.  What would happen?

What was the outcome of this scenario?

The test was discontinued. No reason the DPE would be looking in her logbook as a discontinuance is simply a pause in the testing process. She was eligible to test, she remains eligible to test. If she already told the DPE about this the smart DPE would make a note in her test records explaining what occurred. Once endorsed the applicant is endorsed. If the CFI wants to "revoke" her endorsement I believe it's too late.

The examiner won’t be looking at the endorsement but will look at whether Sara has received the required re-training.
§61.49   Retesting after failure.

(1) The necessary training from an authorized instructor who has
  determined that the applicant is proficient to pass the test; and
(2) An endorsement from an authorized instructor who gave the applicant
  the additional training.

Before the test a new IACRA form must be submitted and signed by an authorized instructor.
The crossed out endorsement is moot. The issue she faces is whether a new instructor would be willing to endorse her for the checkride after just giving training for the failed portion, or whether they would require more from her.

To be clear, I'm asking about discontinuance, not failure. Student Sara passed the oral, but elected not to begin the flight portion because the weather was bad that day.

@SteveV. Sorry, I missed that. Just from personal experience, I did my IFR checkride last month after discontinuing for weather and the DPE did not look at my logbooks again.

The downvote isn't mine. Thank you for the info!


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How does the position of vortex generators on engine cowlings affect flight?
I know what vortex generators (VGs) do. My question is: How come some aircraft have VGs on both sides of the engine, whereas other aircraft have them on the inside only?
For example, in this picture of Boeing 737, a vortex generator is only on the inside of the engine:

On the contrary, the Airbus A319 has them on both sides:

What is The this kind of VG name i mean this VG have special name . Anybody know???

The vortex generator helps only at high angle of attack by energizing the airflow between nacelle and wing. During cruise it will create additional drag without much benefit, especially if the nacelle-wing-combination has been designed well.
However, in the landing configuration it can help to lower minimum airspeed by several knots and make shorter landings possible. Now the aircraft maker has to decide what is worth more: Shorter field length, or slightly less cruise drag.
The Boeing 737 started life with a much more slender engine and had a very short landing gear when more fuel-efficient engines came to market. A lot of brain- and computing-power was needed to develop its pear-shaped engine nacelles, and the vortex generator became an essential element of it. At the inside location it helps to keep the flow path between wing, fuselage and nacelle clear of separated air, whereas on the outside a second generator would make only a minor difference.

So why does the Airbus and the C19 have them on both sides then? The C19 even has them on the outboard engines..

@ROIMaison: I guess you mean the C-17. I am wondering myself - this is a confession of failure on the part of the aerodynamicists. After all, both aircraft were designed from the start with their respective engines. See my comment below this answer.

If it only adds drag during cruise would it be economical to make it retractable like other lift-enhancing devices? That is if there's room inside the nacelle for hydraulics.

@TomMcW: Your "if" is justified - there is little room for a retraction mechanism. This mechanism would need to be redundant to ensure proper operation in case of a single failure - after all, with asymmetric vortex generators the aircraft will behave in funny ways during landing.

To complement previous answer, vanes on nacelles create vortex on high angle-of-attack, which then goes over the wing. This effectively acts as an boundary layer fence reducing span-wise flow over the wing thus improving lift and lowering stall speed at high AoA (is. low speed). The energized flow also improves down-wash in the area of the wing where there is no slat due to engine mounting.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Could a high absolute difference between heading and course give a hint about weather conditions?
I am developing an application that tries to predict the future track of an airplane. At the moment I don't have access to weather data and I am trying to find a metric that could substitute that.
If I am not wrong, a pilot should change his heading according to wind's direction in order to maintain the airplane's course to the target airport. So If the wind benefits the airplane's movement, the absolute difference between heading and course should be zero most of the time. Is that true? If not is any another way in order to get a hint about weather conditions from heading/course information?

Well, how about a little feedback on the answers provided?

Thinking through the possibilities, if you knew the pressure conditions, and the change of those conditions as plane moves, and you knew the wind direction and speed and its changes as the plane moves, you might be able to have some kind of AI software estimate the location of the pressure centers in the airmass you're flying in, and from that calculate the pressure gradients and wind direction/velocities for some distance around the plane's location.  This might be able to produce a "better than nothing" educated guess as to the wind speed and direction ahead based only on the data being received in real time.
To do this, besides an extremely powerful computer (weather forecasting agencies use multiple Cray supercomputers to predict circulation flows, but maybe you don't need that much power) you would need the normal air data inputs like ambient and dynamic pressure (indicated airspeed), and you'd need to know the airplane's true altitude and true position, which can be provided by a GPS interface.
If the software knew the precise height above the earth so it could measure ambient pressure and continuously calculate barometric pressure and measure pressure changes, and it knew the precise location, track, heading, groundspeed, and true airspeed of the airplane so it can continuously calculate wind velocity and direction (FMS systems on jets do that already), it might be able to create its own pressure pattern map of the immediate area, and from that generate a prediction of wind conditions some distance ahead.

A high absolute difference between heading and course can give you a hint of the weather conditions, but that hint is not very useful: it's windy and it blows from the side.
But more seriously though: I think I see what you are getting at, but to predict plane's track for meaningful distances on momentary observations of weather only is not very straightforward. Apart from certain areas in the world, weather usually is not very stable. The algrorithm or neural network doing the prediction would have to constantly update it's "knowledge" of the surroundings to keep up. And it needs a lot of data to figure out that knowledge.
A short version: my less educated guess would be, that asking such an application "check how things are now, and tell me where I will be after flying for 200nm" will yield rather useless answers.
And the longer one: Basic rules to deduce weather pattern from conditions prevailing for, say, 15 minutes should not be all too bad. As JohnK earlier mentioned, you would need multiple data sources: accurate 3d location, airspeed, temperature, heading, pressure, and then measure trends for these. Actually you'd need to know the date too... You'd get a data set that can be used to determine which weather pattern fits best to the conditions (listing the mnemonics on weather patterns would take an hour, so I'll skip that. I've always hated meteorology for that...).
The "fluid" nature of weather is what makes what you are after difficult. If you need a prediction for the track of the plane, it really is not a prediction if it updates constantly.  Predicting track from a single snapshot of condition is highly unreliable for longer distances, and thus pretty much useless, at least in my opinion, and somewhat experience too.

Your question is a little confusing, but let me try to answer and perhaps help you get focused on what you actually need.
First, you are using the broad term “weather” when I think all you really care about is wind.  (Winds are a subset of weather...). So the answer to your title question is yes, a large difference between heading and course indicates a strong crosswind.
As far as the first paragraph, if you have access to the internet you have access to weather data.  Google “winds aloft forecast” and you will see what I mean.  If you are looking for anything more specific please help us by narrowing it down.
As far as the second paragraph, yes... in a direct headwind or tailwind the course and heading will match.  They are only different if there is a crosswind component that needs to be corrected for to stay on course.
Finally, without understanding the purpose or audience for your app, I will just add that every private pilot student by the time they get to cross country flight planning should have a solid understanding of the basic wind triangle.  This means that they can precisely determine where they would end up if they didn’t apply a wind correction.
But that isn’t the point: Predicting the future track due to drift isn’t what is needed (by a pilot...), the correction necessary to prevent drift and determine ground speed is.
But, if you can better help us understand the purpose of this app and the intended users maybe we can provide a better answer.


text:
What is the difference between interference drag and form drag?
What are the differences between interference drag and form drag? And which factors affect them?

Does this answer your question? There are a lot of questions here about drag, have you read through them? Please add some detail about what, exactly, you're confused about, or this may get closed for being far too broad.

Form drag is caused by the body itself travelling through a medium, such as air. The more streamlined the body is, the less form drag it causes. For example a cube has high form drag compared to a droplet shaped body.
Interference drag arises in places where two forms join each other, such as wing - body joint. It is caused by the pressure rises especially on the rear half of both bodies combining and causing earlier separation, as well as turbulence at discontinuities of a shape; the more abrupt the change is, the more interference drag is caused. It can be made smaller by blending the joining bodies into each other with a rounded fillet or such.

Might want to put your last sentence in a comment instead?

Interference causes the pressure rises especially on the rear half of both bodies to combine, causing earlier separation. It is not just turbulence or discontinuities (although those have their fair share, too).

I removed my comment altogether. We'll see how this q makes it....

@PeterKämpf I infused your comment into the answer, could you please see if I got it right. I modified your sentence some, and might have "broken" it....

Looks good. A fillet reduces wetted surface area and draws out the pressure rise. In order to reduce interference drag, it is advisable to separate the areas where pressure rises; that is why horizontal and vertical tails often are not at the same lengthwise station. This also reduces blanketing of the vertical in spins, so it has multiple benefits.


text:
Why use thrust reversers?
I see a lot of planes using thrust reversers and stopping in the middle of the runway in plane spotting videos, but I don't understand why they do it when they can obviously not use the reversers and still stop safely.
Also, doesn't it waste fuel? Don't airlines prioritize fuel savings over stopping faster? Why are reversers used even when they don't need to be used?

Welcome to aviation.SE! We already have a lot of questions about thrust reversers; this one in particular addresses fuel implications. Do they help? If you're new to the site you might also like to check out the [tour].

Can you point to some of the videos? Planes really should not stop in the middle of the runway, they should get off it as soon as possible (and safe). Stopping in the middle of the runway will severely screw up the sequencing of the following aircraft.

@Pondlife I didn't mean a complete stop. I mean they came to like 20-30 knots or so

There's a lengthy paper on the NASA web site that investigated this. There's a lot of detail covered in that paper so I'm not going to reproduce any of it here, but the main positive points that emerge are:

Improved safety margins on landing1
Reduced wear on brakes and tyres
Reduced brake temperature leading to faster aircraft turn around.

Even when setting this against additional engine maintenance and fuel consumption, the airlines surveyed overwhelmingly considered that the installation and use of thrust reversers gave a worthwhile return on investment.
1 Thrust reversers are not considered when assessing braking systems for certification.

Another reason, and I suspect the most important one (though I'm not an airline pilot) is that most terminal buildings are located roughly mid-field.  Therefore, if you can use thrust reversers and slow down enough to exit the runway near mid-field, you have a short taxi to the gate, saving several minutes, which is good for on-time statistics.  Basically the same reason I usually landed my Cherokee rather long when I was based at a commercial field - I could slow, exit, and be right on my FBO's ramp.

@jamessqf The author of the paper covered that. See page 6, para. 2: 'The airlines do not promote the practice of a quick egress from
the runway to meet scheduled arrival times.'

I imagine thrust reversers allow many more movements at busy airports. Reversers get airplanes off the runway much faster, allowing ATC to have shorter spacing between approaching airplanes.

@CatchAsCatchCan: There's a difference between saying "airlines do not promote", and pilots doing it anyway.  Sort of like police departments insisting that they don't have ticket quotas :-(


text:
Is the passenger capacity of the A350 really only 250 pax?
I read online that the capacity of an A350 was 250 passengers, but I took the max cabin width and length of an A350-900XWB, and I ended up with a 400 passenger capacity in a 4-class layout.

Economy was a 3-3-3 configuration, with 2.48 feet seat pitch
Premium was a 2-4-2 config. with a 3 feet seat pitch
Business was a 2-2-2 config. with a full 6 feet seat pitch
First class had a wide 1-2-1 configuration with a full lie flat bed to make for a 7 feet seat pitch

Yet, I still got a capacity way higher than 250 of 234 economy seats, 54 premium seats, 42 buisness seats, and 16 first class seats, and the total for 346 passengers + 8 crew! Yes, I included washrooms for all classes, emergency exits, and the galleys. What does the capacity of 250 really mean?

Airbus themselves give max passenger seating 440, and typical seating 3-class 300-350, for the A350-900. https://www.airbus.com/aircraft/passenger-aircraft/a350xwb-family/a350-900.html
The undeveloped A350-800 is listed with 276 passengers. Maybe that's where you got your 250 number.

Thanks for the response! I always wondered why it could only carry 250,  and now I know that's now true! And love how you included the official Airbus website in your answer.


text:
What prevents a commercial jet from electronically "resetting" the way a computer sometimes does?
I've had a fear of flying for the past several years and one of the ways this irrational phobia manifests is concern over the airplane power-cycling in mid-air: engines cutting out, control equipment powering off, etc.
Occasionally a computer or smartphone will crash and reboot itself, or in a worse case, the power supply or battery will fail and the device will not turn back on.
Does this ever happen with commercial jets? I expect that they are electronically wired up in a fundamentally different way from computers, so that one system failure does not affect the rest, but I've never asked a pilot.

Related : How do redundancies work in aircraft systems?

Related : Why are critical flight computers redundant?

Well, still there are examples of need to "reboot a plane" ;) A350: https://www.theregister.co.uk/2019/07/25/a350_power_cycle_software_bug_149_hours/ and 787: https://www.theregister.co.uk/2015/05/01/787_software_bug_can_shut_down_planes_generators/

Comments are not for extended discussion; this conversation has been moved to chat.

Sometimes they do.  That's (one of the reasons) why planes have more than one.

I'm a programmer and private pilot, so maybe I can help dispel some of those fears.

The computers that run a commercial airplane are conceptually much simpler than the one that runs your phone. This means far less chance of a bug in the software, just because there's less for the programmer to keep track of.
If your phone restarts, it doesn't imperil anybody's life. So, the testing and Q.A. for such a device is basically whatever the company wants to do. On the other hand, computers in aviation are much more thoroughly tested before they can be certified to fly.
Similarly to point 1, the computers on an airplane each have only one job. A lot of the crashes on a typical PC or smartphone come from different apps stepping on each other's toes. (The operating system is supposed to keep each app apart so they can't do that, but point 2 applies to operating systems as well.)
The airplane isn't one single computer, like your phone. Yes, your phone probably has multiple processors, each with multiple cores, but they're tightly coupled together in order to form one computer. The computers on an airplane are networked together, but they are separate computers. If your phone crashes, even if the guy sitting next to you is on the same network, it doesn't affect him, does it? Similarly, if the FADEC for one engine fails (a vanishingly rare occurrence, because FADECs are conceptually among the simplest of computers), all that will happen is that one engine will shut down, with no effect on the rest of the plane.
On the other hand, the really important computers (such as fly-by-wire controllers) have multiple redundancies. So, if one fails, the rest of them can pick up the slack. Even the pilot wouldn't notice except for the warning light that would show in the cockpit.

If you want to see what actually happens when things fail on an airplane, take a look at the following videos:

Complete avionics failure over the ocean.

 The pilot flew the airplane by hand while he shut down and restarted the avionics. The flight then proceeded normally.

Autopilot failure. (Ironically, the same guy as the previous link!)

 The pilot actually let the autopilot have it for a few seconds, just out of curiosity as to where it would take them. But once it started banking more than it was supposed to, he disengaged the autopilot and took over manual control with no issue.

Brand new pilot (less than 60 hours total flying time) suffers complete electrical failure at night

 Engine stayed running despite losing all electrical power to the entire plane. He landed safely.

Edit: Well, just two hours ago as I type this, another video of an in-flight failure just happened to pop up in my YouTube feed:

Generator failure in flight

 He switched to his backup generator, and other than an annoying whine in the headphones, had no other issue with the flight.

Any reason why you included spoilers in your answer?

I, for one, like the suspense of not knowing how each of those would turn out!

@Valay_17 Just in case they wanted to watch the videos themselves without knowing what happens.

@HiddenWindshield Ohh, hah didn’t think of that ...  :)

If it helps, OP, national infrastructure like telecom networks utilitises this concept of redundancy too, as a matter of course. Heck, Stack Overflow utilitises the concept of redundancy - it can fail over to read-only mode in the event of a problem. And those aren't even safety-critical applications! The comparison with your home PC is just wildly off-base.

I believe point 2 and 4 are invalid. 2: Programs are unable to "step on each others toes", even on plain x86 they are strongly separated by protected mode, and you'd need a ring0 exploit to bypass this. The kernel for Windows and Linux is probably tested at least as much as the kernel used in aviation. 4: A typical PC is a set of "networked" computers as well, far beyond your example of multiple cores, there are controllers for everything, many with bus mastering capabilities. Furthermore, avionics systems have strict real time requirements that a typical PC does not suffer from.

Not only are the fly-by-wire computers redundant, they're built using different architectures and programmed by different teams to minimize the chance of a problem affecting all the computers at once.  The A320, for example, has two Intel FBW computers and two Motorola FBW computers.

@AlphaCentauri - here on the Earth we don't need to poke into some other process space to "step on each other toes" - ask some other app to open file with bad data/file name or launch browser to site with 0-day exploit or push accessibility API too far... There are plenty of official IPC communication channels between processes that can be used to cause issues even unintentionally. And here cost of testing to aviation (or any life-critical) standards is way to prohibitive for any general purpose OS to be tested to qualify (at least in default consumer configuration).

An additional reason, which may just apply to the Airbus A380, is that they run software that is partially formally verified, meaning that it is mathematically proven to be free of specific classes of bugs. I believe they use a microkernel called INTEGRITY-178B, a DO-178B compliant RTOS microkernel.

@AlphaCentauri - In addition to what Alexei Levenkov said, a PC is not a "set of networked computers". It is one computer with multiple components. True, it is sometimes possible for a network card, hard drive, etc. to crash without affecting the rest of the computer, but this is rare. If your hard drive goes, you'd better hope that nothing was swapped out of RAM at the time, because if it was, the computer will blue-screen (or the equivalent) seconds later. And if one of the main processors crashes, that's it for the whole system, regardless.

Before we start, it's important to say that your concern is not irrational.  If this were to happen, or if your plane's control systems were to otherwise malfunction in a dangerous manner, your life would genuinely be in danger.
You aren't the first person to have thought of this, though.  For this reason, we have a category of control systems we describe technically as safety related, and there is an entire branch of engineering called safety engineering dedicated to formally assessing these systems and trying to prevent accidents.  This includes airplane control systems, but also anti-lock brakes, medical devices, and any other system where people could be harmed by it going wrong.  The degree to which people can be harmed by this is formally assessed as a Safety Integrity Level based on risk.  The risk is a combination of how likely the event is, how bad the outcome will be, and whether the people involved can take any mitigating action, and it is assessed for every way a safety related system can misbehave.
Note that this assessment may not be as intuitive as you'd think.  I once worked on a chaff and flare dispenser system for military aircraft.  You would think that the risk of failing to fire countermeasures and the pilot being shot down would be your major risk - but the safety assessment (we used an FMEA) showed that the pilot had other mitigating options such as armour and an ejector seat, getting shot down is a chance they'd already accepted when they took the job, and the risk of a crashing plane hitting buildings was miniscule and something that had already been institutionally accepted as part of having an air force.  The most serious risk was actually that the system would misfire whilst an armourer was reloading it, because then they'd get a volley of 36 shotgun shells to the head at close range. The armourer did not sign up to taking that chance, and there was no practical way to protect them.  As a result, our system had to default to not firing if there were any discrepancies.
There are many ways to ensure reliability.  Redundancy is the most popular one.  You can have multiple sensors in multiple locations, so the system can always work out what's going on if one (or more, perhaps) should fail.  There are usually multiple actuators for important flight surfaces, or multiple flight surfaces where the aircraft can remain in control if one or more are damaged.  Passenger planes generally have multiple engines too, and multiple fuel tanks which can be isolated from each other in case of damage.  In a number of cases there may be multiple control systems which "vote" on the right action, so one malfunctioning unit will be ignored.  In the extreme case, each control system may even have been programmed by a different software team, so that a bug in one team's software is extremely unlikely to be present in another team's software.  And there may be other backup systems in place such as mechanical controls.
Another good mitigating method is training.  It's perfectly acceptable for things to go wrong if the people operating it are able to deal with that failure and keep going.  It's important not to underestimate how good people can be.  People can and do also cause failures, so training can also be a case of telling them "don't do that".  Large aircraft are relatively slow to respond to controls, so it's relatively common that pilots can overcorrect and make things worse.  For some commercial aircraft, the standard response taught to pilots in case of instability is to let go of the stick and allow the aircraft to correct itself.
It's worth noting that both these factors are why the Boeing-737MAX disasters are so bad, to the extent that there should be criminal charges brought against the people individually and the organisation collectively.  The system concerned did not use redundant inputs, even though they were available; the impact of the system failing to respond correctly was not assessed nor mitigated; and the crew were given no training in how to deal with its failure, nor even told that it existed.  In the UK, the crime of "corporate manslaughter" exists to prosecute exactly these kind of failures.
The other element to all this though is quality, so that you try to make sure the systems don't go wrong in the first place.  The reliability of software is almost entirely dependent on the quantity of reviewing and testing that takes place.  I'm currently working on software for scientific equipment, and I reckon to spend around 10-20% of my development time on testing.  PCs and mobile phones will be about the same.  When I worked on automotive and aerospace systems, this was entirely reversed - we reckoned to spend around 5-10% of our time on coding, 10-20% of our time on design, and the rest of our time went on reviewing and testing.
Change control is also radically more locked down.  Microsoft may release an upgrade and then do damage control on the few cases where it misbehaves, and sneak in a few extra features at the same time.  In safety-related development though, you don't change a single line of code without formal sign-off that (a) everyone understands what that change will do, (b) that this change fixes this bug and does not change anything else, and (c) that this change is even needed.  Many bug triage sessions involve us spotting bugs where we eventually decide that the impact of the bug is tiny (perhaps we're 10ms later turning on a warning light for example), but the risk of trying to fix the bug could potentially be high if we happened to get it wrong, so it is safer for this trivial bug to stick around.
As the Boeing-737MAX case shows us, all these processes are only worth a damn if people follow them.  The processes exist though, and they are best practise in an industry of tens of thousands of engineers worldwide which has plenty of formal standards internationally to establish this.  Failing to follow these standards is almost by definition gross negligence, and most countries have laws which allow prosecution of people and companies who are negligent to this degree.  Most engineers would like to do a good job anyway; but the laws ensure an organisation as a whole stays honest and doesn't cut corners.

Very good answer. You could, if you have time, elaborate a sentence or two on how the very processes of software and hardware development for safety critical systems are strictly defined to ensure the safety of the finished product, e.g. as in IEC 61508 and then domain specific norms. To actually have a proven, systematic, documented process which ensures that requirements are fulfilled, specifications are followed, reviews and testing is ensured etc. is the most crucial difference to general product development.

@Peter-ReinstateMonica Thanks. All very true, of course.  I was worried I'd gone on too long as it was though! :)

It might be worth noting that the test / quality process requires independence from the team that actually implemented the code in many cases. (Certainly true for DO-178 and DO-254).

"The most serious risk was actually that the system would misfire whilst an armourer was reloading it, because then they'd get a volley of 36 shotgun shells to the head at close range" - Why was the armourer standing in the flare dispenser's line of fire while reloading it?

@Vikki The design of the dispenser is basically a 6x6 grid of tubes in a rectangular frame. A flare or chaff cartridge goes in each hole, and then the armourer plugs the loaded frame onto the mounting panel which has the firing contacts. Inevitably this means he has 36 armed tubes facing him as he slides it home.

@Graham: That seems like... not the best design.

@Vikki It's the only practical design, given how you need the things to be fired out.  And like I said in the post, it's safe enough if you make damn sure the firing contacts can't be live when they're doing it.

Your concerns are reasonable and justified. A mid-air shutdown or reboot would be catastrophic to an airliner. Which is why, engineers designed the systems such that this scenario is practically impossible to happen.
Electrical power
An airliner has multiple electrical power source. Each jet engine has a built-in generator. When the turbine spins, electricity is generated. Each generator can be independently turned off should a problem arise. Most airliners also have an Auxiliary Power Unit, or APU. The APU can be started in an emergency to provide backup electrical and hydraulic power to the airplane, as done in the famous Hudson Riving Landing.
If everything fails (for example if the airplane runs out of fuel), limited electrical power can be provided by windmilling, either using the Ram Air Turbine (e.g. Boeing 777) or by windmilling the turbines themselves (e.g. Boeing 747), as the airplane slowly glides towards a landing spot.
Then there is the battery, of course, which is charged at all times. It can provide limited power in case of emergencies.
Computers
All airliners come with multiple flight control computers. The units are built by different manufacturers, on different CPU architectures and different source codes. The chance of all units failing at the same time due to a bug or defect is very low. In the unlikely event that one of the units fail, the pilots can disconnect that unit from the rest of the system.
For example, the Airbus A320 has 2 Elevator Aileron Computers, 3 Spoiler Elevator Computers and 2 Flight Augmentation Computers. Each unit can be disabled shall it malfunction.
Mechanical linkage
In the extraordinary unlikely event that electrical power is completely lost, certain flight controls are linked to the cockpit via mechanical means and can be operated with human force. For example, the emergency procedure for a complete flight computer failure in the Airbus A320 is to land the aircraft using nothing but rudder peals, the elevator trim wheel and throttles. This has never happened in history.

Might I also add that long oceanic flights such as London to New York would have to be ETOPS certified (plane, crew, airline, etc). This ensures there is always a palce for emergency landing at any point in the flight if something still goes wrong.

as somebody who did software tests on an unimportant (class D, will explain soon) system for an airplane to be approved to be landed on civilian airports: In airplane there is a strict hierarchy on what kind of software functions mean; they are listed in DO-178B.

Class A systems are assumed to be "failure free"; they are extremely well tested. These systems are not meant to reboot, and they typically will not turn off or do any other additional action upon an error condition. (e.g. when an engine controller looses the connection to the flight deck it will just remain in it's last engine setting). Class A systems are developed under an high amount of testing and level of scrutiny.
...
for the system (class D) which I tested the main logic was "if there is an error, send an error message and then get of the network, halt and wait for a reset from the cockpit. Even Class D system tests include test procedures which are not used often (e.g. white-box tests with hardware emulators). These are still the best tested software which I have seen in my life.

The logic here is that an unimportant systems crash will never hamper with the important systems function (the pilot can choose when to rest these). Most systems in an airplane are double redundant. The micro controllers and HW architecture used are designed in a way that simple failures on a board will be limited in impact. The main network (e.g. AFDX) is also redundant, and measures are taken in the interface that software running wild does not go outside its bound in using the busses.
Normal reset procedures are safe in respect to leaving the plane always in a controllable state. An example of a wrong reset procedure  - powering down both flight control computers, which is not allowed while in air, because the pilot was not happy with the results of the standard way of resetting the computers - was Air Asia Flight 8501.

Occasionally a computer or smartphone will crash and reboot itself

This can happen for two reasons: a software error or a hardware error. Both can cause the CPU to stop processing new instructions (i.e. a "hang") or to cause the machine to reboot itself. The latter is close to a hang, because the Operating System detects it cannot continue operating normally and issues a hardware restart.
The possible causes are endless, but the results come down to the same: the processor cannot execute any new operations and therefore not continue operating normally. This is suboptimal if human lives depend on its continuous functioning.
Hardware errors can be caused by degraded functionality, through damage or wear. For example a power supply that cannot deliver the required power at all times, or a memory module that is damaged through electrostatic discharge, causing random bits to "flip" (a 1 unintentionally being read as a 0 or vice versa).
Software errors are caused by programmer errors or installation errors. A clean Operating System installation (Windows, Linux, MacOS, ...) on your computer or smartphone, having well-functioning hardware, given the hardware is supported by the OS and the appropriate drivers are installed so the OS can communicate properly with the hardware, will not crash. Sure, decades ago some OSes were prone to crashing after being up for a certain amount of time, but it's 2020 now. Those issues have all been ironed out of modern operating systems.
The problem with consumer-grade hardware and software is that it's not life-critical, not redundant, and people want to be able to install random applications on their devices, distributed by random software developers. You won't see a pilot opening the App Store on your Airbus mid-air and install the new Christmas Lights app to let the cabin lights blink festively, which just happens to stop the fuel pumps because the developer never tested it in flight.
So how do airplane manufacturers stop this from happening:

Redundancy: when one system stops (or its outputs lie outside valid values), there's a reserve system to take over.
Specificness: as opposed to general purpose computers, the devices in an airplane have a very specific goal, and are built, installed, configured and tested for that goal.
Testing: you might bring your computer or phone to the shop for maintenance when it starts behaving erratically. That might be too late for that hardware, but usually they'll be able to recover your pictures. Buy new hardware and/or reinstall the OS and you're good to go. Planes are checked more regularly.

I love reading this Stack Exchange, but I don't know much about planes. I do know a thing or two about computers though, so I hope this answer is useful and on-topic.

Modern operating systems, like Windows are not used in flight computers. Linux might be used, but I think it's rare (because of the modifications needed to meet avionics standards). More commonly would be LynxOS or VxWorks, which are highly specialized systems designed to run on high-reliability avionics and other industrial systems, and aren't a traditional OS like you would think of Windows or Linux.

@Ron that was absolutely not what I wanted to imply, sorry if so! "Rebooting engine control for Windows Update..."

@RonBeyer - apart from VxWorks cert, Greenhills integrity is also used in safety critical avionics applications. vxWorks is also very popular in avionics for mission critical applications.

A desktop tends to hang on a crash so the user / developer can see that and maybe gain some info about why it crashed.  (Or at least the fact that it did crash).  When you have other requirements that take priority (keep operating) like many embedded systems, you include a watchdog timer that reboots the system if the OS doesn't poke it every millisecond or whatever.  Embedded systems typically don't take long to boot up.  A classic example of something similar is Did the 1202 error and associated reboot prevent disaster on Apollo 11 landing?

Regarding power cycling specifically (as opposed to software-based failures in general which other answers have detailed very well), it's not a problem at all. Unlike a typical PC or smartphone which takes time to turn back on and can lose data when power is lost, the control systems on an airplane are typically designed such that they will resume complete operation the second the power is restored.
Think of it more like your refrigerator internal temperature monitor than your personal computer. If you power cycle it, it will immediately begin functioning again as if nothing happened.

In particular, apart from the auto-pilot and the navigation system, pretty much none of the systems are actually reliant on any form of storage, not even RAM. They deal with real-time sensor data. If they reset, no data is being lost, because they only work with data that is being captured in real-time anyway. And a misbehaving auto-pilot or NAV doesn't make the plane fall out of the sky, it just means that responsibility for where you are going and where you are now moves from the computer to the pilot.

@JörgWMittag Well strictly speaking, they all have RAM, even if it's only a few kilobytes. While it's possible to run a program without any RAM, it's too limited even for these purposes. Doing calculations on real-time sensor data, or even non-trivial arithmetic, requires more than just a few general purpose registers.

Reboots can also sometimes be acceptable if you can reboot quickly without losing critical information.  This occurred as part of the Apollo 11 landing when they had the 1202 alarm:

He realized that the 1202 was a code meaning that the guidance
  computer on-board the landing craft was getting overloaded with tasks.
  The programmers had anticipated this overloading might someday occur,
  and so had established a system internal aspect that would
  automatically do a fast reboot and then a memory restore to try and
  get the computer back underway.

but this would not at all apply to the fly by ethernet controls that operate the actual control surfaces, surely?

If an error is detected, it is better in many cases for the system to be inop temporarily while it reboots rather than be stuck in a faulted state.

@Fattie sure it would. Better to do nothing for a moment, then do the right thing, than to do the wrong thing immediately.

This seems more like a comment, since there's really no comparison between an ancient computer with magnetic core memory and rope memory and exceptionally limited computing requirements, and a complex modern machine with hundreds of microprocessors with incredibly complex ISAs and millions of lines of code written in multiple programming languages.

@forest: Modern embedded systems do still have a watchdog timer that would reboot if the system locked up, instead of just sitting there like a desktop so a user can write down the error code (or notice that it crashed in the first place).  But yes, the quote in this answer makes it sound like a primitive garbage-collection scheme.  However, the Apollo 11 AGC did have a watchdog timer (they called it the "Nightwatchman" :P, and was one of a few things that could trigger a reboot:   How did the Apollo guidance computer handle parity bit errors?)


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What is this mysterious thing on the vertical stabilizer of the Boeing 707?

Have you ever seen this mysterious line-shaped thing on the vertical stabilizer of a Boeing 707 or 720? What purpose does this thing do?

Could you  maybe provide a picture that is a reasonable size?

It is the high-frequency shortwave antenna of a 707-321B, specifically Clipper Yankee Ranger N418PA. Also present on the KC-135.
You can read about the Antenna Coupler Program from the people involved in its development.
You might want to look at What radio frequency bands are associated with the long wire antennas, why do some aircraft have them and others don't? for more information.

source

source

Surely UNIVAC Coupler isn't the mainframe?

The Univac division of Remington Rand Corp were the creators? developers? designers? of the UNIVAC computer. A few years later they developed electrical couplers for military aircraft antennae, which were later made available to civilian airlines.


text:
When does the 18-month time limit begin for the JAR ATPL examination?
Here is an extract from JAR-FCL 1 concerning the period during which it is necessary to pass 14 ATPL exams:

Subject to any other conditions in JARs, an applicant will be deemed
  to have successfully completed the required theoretical knowledge
  examination for the appropriate pilot licence or rating when awarded a
  pass in all of the required subjects within a period of 18 months
  counted from the end of the calendar month when the applicant first
  attempted an examination.

So taking this into account, it is possible to have 19 months to pass all the remaining ones (13, in case of a success during the first sitting) after first exam sitting in reality if you sit it on the first day of the month. For example, if you take first exam on the first of March, during the remaining part of this month the clock isn't ticking and will start doing so only on the first of April, so you get an additional month. Am I right?

Geez... In in Canada it's 2 exams, 80 questions each.

Dont plan on pushing the envelope. You wont like the result when you do that in real aircraft.


text:
Why the "Terminal" in Terminal Aerodrome Forecast?
Why isn't it just called Aerodrome Forecast? Where does the "terminal" part come from?

Because aerodromes are where flights terminate.

Terminal refers to the airspace near the airport, more or less similar to the ATC usage of terminal as in "terminal airspace" or area. TAFs cover the airspace in a 5 mi radius of an aerodrome, the immediate terminal area. The T in METAR also refers to terminal as in "Meteorological Terminal Aviation Routine Weather Report".

Also in ATIS and FAA is using Area instead of Aerodrome sometimes.


text:
Why not add more blades to a helicopter?
There's something that bothers me inherently, and just like the Hueys from Vietnam, 'only' having two blades.  Doesn't seem enough, I mean, if despite your best planning, engineering -- whatever -- you lose one, you're like a twisty brick plummeting to the ground; and on the way down, it won't be any value shouting "That shouldn't have happened!!"
OK, first up, I accept the higher centre of gravity thing, if I have 4 blades, f'instance, but that's an engineering problem that can be fixed, ie. that's no reason.
There must be some other reason I can't have -- if in fact I actually can't -- have 4 or 6 or... isn't there an Aerospatiale chopper with 9??
Please tell me what I don't understand....

Welcome to Stack Exchange! This is a good question...

Actually the most efficient would be to have a single blade. Multiple blades are always working in the wake of the leading blade.

I should have said... I was actually thinking in the context of gyrocopters / gyroplanes.
Yes, I  can see how a blade would be working in the wake of the one ahead of it, but I'm going to assume (obviously can be wrong) that a 4-bladed configuration is sufficiently separated for that not to be a limiting factor... for gyrocopters.

@ymb1 I would even call the second one a dupe. Peter's answer in particular covers most of what there is to solidity factor selection.

@SteveHlavenka Have a look at his answer: https://aviation.stackexchange.com/a/32252/23223 If that doesn't make it clear for you, could you explain what benefit do you expect to obtain by having more blades?

Does this answer your question? Why are three-bladed helicopters relatively rare?


text:
What is the answer to this question about aircraft position based on an NDB?

An aircraft is intending to track from NDB ‘A’ to NDB ‘B’ on a track
  of 050° (T), heading 060° (T). If the RBI shows the relative bearing
  of ‘A’ to be 180° and the relative bearing of ‘B’ to be 330° then the
  aircraft is:

A. Port of track and nearer ‘A’
B. Port of track and nearer ‘B’
C. Starboard of track and nearer ‘A’
D. Starboard of track and nearer ‘B’

Think of it this way: the plane's heading is known, but the plane's actual track is not. Also the bearing from A to B is known-- it is the intended track.
Bearing from point A to plane = reciprocal of 60 + 180 = reciprocal of 240 = 60.
Bearing from point B to plane = reciprocal of 60 +330  = reciprocal of 390 = reciprocal of 030 = 210
Draw a sketch of A and B, with B 50 degrees from A.  Draw the bearings from A and B to the plane-- the plane is located where these bearing lines cross-- and it will be clear that the plane is right of the intended track, and closer to B than A.
See diagram in other answer!
Whichever corner of the triangle is the larger angle--  the corner by point A or the corner by point B-- must correspond to the waypoint that is closest to the plane.

Thanks alot , when comparing both answers the problem becomes very easy.

Yes, "D" is correct. A picture is worth a thousand words......


text:
Is "takeoff distance" measured over the ground, or to a point in the air?
I read that takeoff distance was the distance for which the airplane is on the runway, as well as the distance for which it ascends to 15 meters off of the ground.
Does it get measured as the hypotenuse of the "triangle" the airplane makes, or as the projection of its movement onto the runway, which is a purely horizontal distance?

Welcome to aviation.SE! Are you asking about a specific country or regulator? There's a regulatory definition in the US (for transport aircraft) but you mentioned meters, so that might not be relevant to you.

I was trying to understand the general concept, so the exact standard doesn't matter too much, just how the measurement is taken. Thank you !

I think for basically any airplane for which takeoff distances are specified by the manufacturer (i.e., ignoring souped-up bush planes), the difference between "projected" horizontal distance and actual flight path distance is going to be negligible. In mathematical terms, sqrt(a^2 + b^2) ~= a when a >> b. That said, it's the horizontal distance that matters as explained in the answers.

Takeoff distance will always be measured horizontally along the ground, because it is used to calculate and reference required runway length.
Also, I should point out that while 15 meters (50 feet) is common in some regions, other requirements exist in other regions and under certain circumstances. For example, in the U.S., according to CFR §25.113, a height of 35 feet (10.7 meters) is also used.

Takeoff distance is always measured as horizontal distance.
Under 14 CFR Part 23, which apply to aircraft seating 19 passengers or less and a maximum takeoff weight of 19,000 lb or less, the pre-Amdt 64 specifications are:

For multi-engine jets of 6,000 lb or more, or commuter aircraft, 23.59 applies. Takeoff distance is measured as the horizontal distance between start of ground roll to where the aircraft attains 35 ft off the ground.

For others, 23.53 applies. Takeoff distance is measured as the horizontal distance between start of ground roll to where the aircraft attains 50 ft off the ground.

Note that a lot more nuance is involved since the greater of one-engine-inoperative distance and that with 115 percent of all-engine-operating distance goes into the actual performance number.
For Part 25 aircraft (transport category), there is a further distinction between dry and wet runway (25.113):

On a dry runway, it is measured to 35 ft.

On a wet runway, it is measured to 15 ft.

This information is harmonized with other regulatory agencies, such as EASA and TCCA.


text:
What exactly is an ATC clearance and how is it requested?
What exactly does it mean to get an ATC clearance?
I’m asking the question because I read point 8015 of the Standardised
European Rules of the Air (SERAs), which states that ATC clearances are requested by submitting a flight plan. I always thought that this was done by a radio-request when asking for start-up (at least for IFR-flights)...
And what does it exactly mean to get certain ATC instructions during the flight (for example in the case of vectoring)? Are these amendments to the initial clearance or do I mix something up?

I realize that the question was asking about EASA, but perhaps an FAA perspective will increase understanding.  The FAA Pilot Controller glossary defines clearance as:

AIR TRAFFIC CONTROL CLEARANCE [ICAO]− Authorization for an aircraft to proceed under conditions specified by an air traffic control unit.

Note 1: For convenience, the term air traffic control clearance is frequently abbreviated to clearance when used in appropriate contexts.

Note 2: The abbreviated term clearance may be prefixed by the words taxi, takeoff, departure, en route, approach or landing to indicate the particular portion of flight to which the air traffic control clearance relates.

In short, clearance is permission, and is generally associated with operating under IFR in controlled airspace.
I will disregard taxiing at a tower controlled field because it can be done under IFR or VFR.  Additionally, I have heard compelling arguments that you are not “cleared” to taxi, but are given taxi “instructions”.  (Ground controllers will never use the word “cleared” when telling you to taxi).
But that is a separate discussion...
The process begins with filing an IFR flight plan.  This notifies ATC of your intentions, and gives them time to enter your request into the system.
The actual clearance is issued when you call them, (radio at towered field, land line at uncontrolled field) field and put your clearance “on request”.  When you call, ATC will issue the clearance.  It will be either what you filed for, or it may contain amendments.
In the clearance you may be told to expect radar vectors.  This then becomes part of your clearance.  However, vectors assigned as needed during flight for traffic separation aren’t generally considered an amendment to the clearance.  Think of them more as a temporary detour around something.
For example, if ATC needs to amend your clearance they will tell you specifically that they have an amendment, and ask when you are ready to copy.  The amendment will generally be a rerouting that you are expected to follow on your own, and may also include a different altitude.
If given a vector, however, the expectation is that is it a temporary heading needed by ATC to provide traffic separation.  Once the need for positive ATC control is past they will clear you to resume your own navigation along your flight planned route.

These days the word “cleared” is reserved for use with take-off and landing, so taxi instructions not using it does not imply it is not clearance. Departure, en-route and approach clearances won't include that word either and they definitely are clearances.

@Jan Hudec, good point, I seem to recall that distinction about not using the term on ground control frequency.  However, at least in the US, the word “cleared” is always used for departure, enroute and approach clearances.

For departure? That one is received on the ground, so it definitely shouldn't use the word – confusing departure and take-off clearance was the cause of the worst crash ever and the reason why use of the word was restricted.

@Jan Hudec, hmmm... another good point.  I have only ever heard the departure clearance embedded with the rest of the route clearance.  I.e.  "Cleared to XYZ via the ACE1 departure, climb to..."  I assume you are referring to Tennerife?  It has been a while since I looked at that one but I seem to recall they issued or amended the departure instructions on the ground.  And maybe while he was in position for takeoff?  Lots of things wrong with this, but context and timing are important too.  There isn't much risk in being cleared for a SID before you even taxi.

Now that I think about it, following a runway change I have gotten an amended departure clearance while I was taxiing.  I don't recall ever being confused whether or not I was cleared for takeoff though.  Unambiguous concise communication is the key:  "hold short runway 32, I have an amendment to your departure clearance, advise when ready to copy..."

@JanHudec "Cleared" is definitely used for more than takeoffs and landings, at least in the US. IFR route and departure clearances ("N12345 cleared to..."), approaches ("N12345 cleared ILS 31 approach"), VFR traffic entering Bravo airspace ("N12345 cleared through the Bravo airspace") and there are probably more I'm forgetting. And you're correct about taxi clearances: the orders 3-7-2 say not to use "cleared" for taxi operations.

An ATC clearance cover a wide variety of approved requests by air traffic control.  The basics are it’s granting permission to a pilot by that controlling agency to fly through airspace or conduct a flight by a filed flight plan under the control of that agency or to move an aircraft on the ground through a controlled movement area on the surface of an airport.
ATC clearance are granted as I said by the appropriate controlling agency.  Typically an ATC request is made using two items:  The callsign of the aircraft making the request and the specific request.
Examples:
“Savannah Ground, Cessna six tree tree seven X-ray on the ramp at Signature, request taxi VFR with Information Charlie.”
“Portland Clearance Delivery, Lear two fife seven tree Victor at Flightcraft, ready to copy IFR to Centennial with Information Papa.”
In both cases, the flight crew identified who they were and what they were requesting.  ATC will respond with both the callsign of the aircraft and the specifics of the clearance they are granting.
Examples:
“Cessna six tree tree seven X-ray, Savannah Gound, Taxi to runway one niner via Bravo, hold short of Runway two eight.”
“Lear two fife seven tree Victor, Cleaed to Centennial via Whamy Four departure, Kimberley transition, then as filed.  Climb and maintain one fife thousand, expect flight level four one zero one zero minutes after departure.  Departure frequency on one one eight point one, squak fife four four seven. Contact Portland Ground on one two one point niner when ready to taxi.”
Once this has been done ATC has now granted the clearance.  Pilots are generally expected to read back clearances to verify they understand and will comply with the specifics.

As Carlo so well put it, ATC clearance are given for many different reasons and scenarios. Even an aircraft flying VFR or one not on a flight plan will need to get ATC clearance to do many things such as:

Entering and being in a movement area.
Especially entering or being on a runway or taxiway.
Takeoff and land at an airfield with an operating control tower.
Flying IFR.
Flying in Class A, B, and some Special Use Airspaces.
Even flying in Class C & D airspaces since maintaining two-way communication could be interpreted as having clearance.
Approaching the airport either on an Instrument Approach Procedure or joining the traffic pattern at a towered airfield.
Etc.

In the US, you can request clearances simply by utilizing radios (NavCom, CPDLC, et.), telephones, Ground Communication Outlets, transponder codes, maneuvering to receive light-gun signals, etc. Filing a flight plan will notify ATC of your intent to request a clearance and the details of your intentions. It will set expectations for/of both you and ATC. You will still have to request and receive your clearance through the aforementioned ways. Although a flight plan is not mandatory (but recommended, none the less) for VFR flight in the US. Filing, or the lack there of, of a flight plan does not preclude requesting and receiving clearance in the above mentioned situations.
Where it is mandatory, it will still be mandatory without a flight plan. And, the flight plan itself is not considered the request nor the receipt. Although Pre-Departure Clearance and Automated Digital Departure Clearance procedures and systems make it possible to request and receive clearances without actually talking to ATC. In the future, the two-way digital link of ADS-b might make it possible to receive clearances and separation instructions digitally through those devices as well.


text:
How does one specialize in a certain sub-field of aerospace engineering?
I want to become an aerospace engineering but want to be specialized in Aerodynamics. How do I do this in college?

You would study aerodynamics, it's that simple. Some constructive criticism here, you have several questions here about aviation careers, and have already received replies and helpful comments even on the ones that have been closed. They are all summed up by 'study your ass off, get very good grades, learn a crap-ton of math, and get into a really good university with an actual aerospace engineering program'. There's no magic here, and no easy answers.

Ok thanks! What math would you say is the most important to take?

The math courses required for your degree.  Reach out to a couple university aero engineering departments. Start with their web sites. There are plenty of people there that will gladly explain their programs and what it takes to 1. get in, and 2. graduate.

Find out what colleges offer Aero as a major, and set your sights on those.  Not everybody does, but the ones that do are the ones that will do what you're looking for.  All of that being said, college counseling isn't really on-topic at Av.SE.

@LukeJustin I truly have the answer on "which math".   ALL MATH.

I'm going to speak for a North American post-secondary experience. The simple answer is, you don't get to specialize in any particular domain within aerospace engineering as an undergraduate, even if your major is aerospace engineering. Why? Because you don't know anything yet for you to specialize. How can you know what you like, if you don't know anything yet?
The first two years of college are foundational studies, during which the courses and materials are more or less the same across any engineering field. You mentioned aerodynamics. Well, you can't do aerodynamics if you don't know linear algebra, vector calculus or basic fluid dynamics. The last two years would be more tailored to your major, but these will only give you a general gist of all the major domains that aerospace touches.
You get to specialize once you enter industry, or if you choose to pursue even higher educations (Master's, PhD, professorship). Whatever it is, what you ended up specializing will probably be very different than what you thought you'd be doing at the start of college, either because of change of interest (now that you know what the fields actually are), or monetary reasons (field A is more lucrative than field B), or pragmatic reasons (because field A has N openings and field B has zero).

That last set of reasons is very real.  You go where here's a job.  I spent 4 years of college thinking I was going to work in airplane development.  I ended up spending the first 9 years of my career in launch vehicles - a rocket 'scientist'.  Some of the most fun jobs ever.  The last 25 years have been in avionics, various comm, nav and surveillance systems.  Never could have imagined the path I took and where I am now (except my soon to be next job - retiree.)